In a large motivated and skilled group (such as the pro peloton) it's amazing how little you have to do as long as you're on the right wheels. I remember reading that on one the flat TDF sprinter stages a few years ago the average pace was 44kph but one of the mid-pack wheel suckers (probably a domestique on a day-off) only averaged 150 watts of power output.... my gran could manage that! That said, it takes an immense amount of skill to sit in a pack like that and it all changes once the road kicks up.